Write tests for all custom Avocet widgets

[bertpareyn]

Tests should be written for all widgets that are custom to Avocet.

• [ ] Addendum Link
• [ ] Avocet Floating Navigation
• [x] Avocet Footer
• [x] Avocet Header
• [ ] Confirmation
• [ ] Modal Addendum
• [ ] Modal Terms
• [ ] Modal Upload New Version
• [ ] My Uploads
• [ ] Publication File
• [ ] Publication Form
• [ ] Publication Metadata
• [ ] Submit Publication

Note*: I haven't checked if all of these are still used.

[bertpareyn]

Avocet footer and header test at https://github.com/CUL-DigitalServices/avocet-ui/pull/374